This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
3GPP LTE radio interface offers high peak data rates, low delays and increase in spectral efficiencies. LTE ecosystem supports both Frequency division duplex (FDD) and Time division duplex (TDD). This enables the operators to exploit both the paired and unpaired spectrum since LTE has flexibility in bandwidth as it supports 6 bandwidths 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz.
One interesting scenario for deploying LTE in Wireless Communications Service (WCS) band. It is the requirement from FCC to have each UE has a maximum duty cycle of 25% in bands A/B and 12.5% in bands C/D. For a TDD system the requirement from FCC is that maximum duty cycle allowed is 38%. Our simulation results and analysis showed that LTE-TDD is a good candidate for deployment compared to LTE-FDD with UE duty cycle limitation.
Since TDD operation involves one single carrier frequency for both uplink and downlink transmissions, both the base station and the UE must switch from transmission to reception and vice versa. Table 1 shows the transmission format according to the 3GPP standard.
TABLE 1LTE-TDD transmission formats per frame of 10 msDownlink-to-UplinkUplink-Switch-downlinkpointSubframe numberconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
One fundamental difference between FDD and TDD is that there is no one-to-one association between uplink and downlink in TDD. The other main difference between LTE-FDD and LTE-TDD is related to the transmission of HARQ acknowledgements. For FDD, the acknowledgement of data received in a subframe “n” is transmitted in subframe “n+4”, while for TDD, the acknowledgement obviously cannot be transmitted until an uplink subframe occurs. For the uplink heavy (configurations 0, 6) and the symmetric uplink/downlink configurations (configurations 1, 2) each downlink subframe has been associated with an uplink subframe in such a way that acknowledgements from at most one downlink subframe needs to be transmitted in every uplink subframe. However, in configurations where the number of downlink subframes per radio frame larger than the number of uplink subframes (configurations 3, 4, 5) reception of several downlink subframes may need to be acknowledged in a single uplink subframe.
3GPP LTE offers two mechanisms to handle this:
A. ACK/NAK bundling
B. ACK/NAK multiplexing
Determining which mechanism to use is configured by the network on a per UE basis. If the UE is allocated ACK/NAK multiplexing then a lot of control signaling is needed for each DL transmission, thus reducing the UL throughput (Coverage). On the other case, if ACK/NAK bundling is used, then DL throughput is affected for some UE's.